Putting the BUFG=CLK/OE/SR property on internal BUF and CKBUF symbols will cause the fitter to issue a warning xr5100. It inserts an OBUF for BUFG=CLK on BUF or CKBUF symbol, and for BUFG=OE on BUF symbol. The output PAD is generated with a name consisting of the BUF/CKBUF's input net name appended with "/OBUF" (not a user signal name). It also disconnects CKBUF with BUFG=OE and trims all logic.
1. Use BUF components instead of CKBUFs.
2. There is no known way of getting the fitter to not insert an OBUF.